1. Field of the Invention
The invention relates to narrow bandwidth lithographic exposure systems such as excimer and molecular fluorine laser systems, and particularly including a line-narrowing resonator configuration including fixed or adjustable wavefront curvature compensation.
2. Discussion of the Related Art
Line-narrowed excimer lasers are applied in the field of photolithography for production of integrated circuits. Achromatic imaging optics for this wavelength region are difficult to produce. For this reason line-narrowed excimer laser radiation is generated for use in photolithographic applications in order to prevent errors caused by chromatic aberrations. Exemplary bandwidths for different imaging systems are tabulated in Table 1 for the excimer laser wavelengths 248 nm (KrF laser), 193 nm (ArF laser), and for the molecular fluorine laser wavelength 157 nm (F2-laser).
TABLE 1imaging optics248 nm193 nm157 nmrefractive optics:0.4-0.6 pm0.3-0.6 pm0.1 pmcatadioptics 20-100 pm10-40 pmapprox. 1 pm
Narrow band excimer lasers such as ArF lasers emitting around 193 nm and KrF lasers emitting around 248 nm, as well as in the near future F2 lasers emitting around 157 nm, are used in the semiconductor industry in the production of the integrated circuits to make structures at or below 0.25 μm, such as around 0.18 μm or below. Even smaller structures will be produced used extreme ultraviolet (EUV) exposure radiation sources such as generating radiation at wavelengths between 11 nm and 15 nm. To prevent imaging errors caused by chromatic aberrations, radiation of narrow bandwidth, i.e., less than 1 pm and particularly in the range 0.3-0.6 pm, may be used. Another important laser beam parameter is the spectral purity, which is related to the bandwidth and is defined as the spectral range that contains 95% of the output pulse energy. New high numerical aperture (NA) imaging optics used in the photolithography work with exposure radiation having a bandwidth of less than 1 pm.
Current lithography lasers operate at repetition rates typically of up to 2 kHz. To produce higher throughput, it is desired to operate these lithography lasers at higher repetition rates such as 4 kHz or more (e.g., even 10 kHz or more). The averaged power incident upon optical elements within the laser cavity generally increases as the laser is operated at higher and higher repetition rates, and will rise by a factor of two or more when the repetition rate is increased from 2 kHz to 4 kHz or more. A very high thermal load on intracavity optical components, especially of narrow band optics such as prisms or etalons, can cause undesirable wavefront distortions in the laser beam, even at 1-2 kHz, and especially at higher repetition rates. These wavefront distortions are typically caused by thermally induced changes of the refractive indices of materials of the intracavity optical components resulting in time dependent variations of the spectral distribution of the laser beam, and of near and far field intensity distributions. It is desired to have a high power laser, particularly for photolithographic applications, wherein effects of wavefront distortions are reduced or prevented, such as by providing a resonator having intracavity wavefront correction or compensation.
A typical line-narrowing arrangement used in the excimer lasers, e.g., as set forth at Rückle et al. Optics and Laser Technol. 19, 153-157, 1987, includes a high order echelle grating, a prism beam expander and an output coupling partial reflectivity mirror. The wavelength of the laser can be tuned by turning the grating, a folding mirror when a Littman configuration is used, or one or more of the prisms in the beam expander. The wave front of the radiation in front of the grating typically deviates from the flat due to distortions in the resonator optics. This can generate a spectral degradation of the beam and an increase in the laser output beam bandwidth.
As mentioned, another important beam parameter is the “spectral purity” or the wavelength interval that contains, e.g., 95% of the energy of the laser radiation. Excimer lasers may include a planar diffraction grating within its resonator for providing dispersion as a wavelength selector for narrowing the bandwidth of the laser oscillation. To increase the resolution of the grating, a beam expander may be used to reduce the beam divergence. Even when a beam expander is used, the wavefront of the beam in front of or incident upon the grating is generally not planar. The radius of curvature of the wavefront may instead depend on the magnification of the beam expander in the region of, e.g., 400 m . . . 1200 m. The curved wavefront results in a broader spectral linewidth or bandwidth due to the fact that different portions of the curved wavefront strike the planar grating at different angles.
It is recognized in the present invention that it is desired to solve the above-described problem by providing an excimer or molecular fluorine laser resonator having wavefront compensation, and preferably including adjustable wavefront curvature correction, so that the incoming beam wavefront substantially matches the surface of the line-narrowing grating or other dispersive element such as a prism or an interferometric device.